Touchscreen panels (TSPs) are widely used in consumer electronic devices. Resistive and capacitive touchscreen panels have widely adopted as input means for data processing devices, in particular for mobile devices such as smartphones, tablet computers and laptops. There is also growing interest in pressure sensing touchscreen panels, for example as described in US 2014/0008203 A1.
Regardless of the mechanism used by a touchscreen panel to detect user inputs, mis-registration of user input is undesirable. For example, mis-registration of the presence or location of a user's interaction using a digit or stylus. There is also a need to keep the power consumption of touchscreen panels low to avoid shortening the battery life of mobile devices. Two of the main factors which influence touch registration and power consumption are noise and multi-valued offsets in the output signals from a touchscreen panel.
Touchscreen panels are commonly arranged to record user interactions or touch events in a two dimensional grid or array arrangement. As used herein, the term “touch event” refers to a user interacting with a touchscreen by touching or pressing the touchscreen panel using a digit/finger or a suitable stylus. Herein, the signals output from a touchscreen panel having a two dimensional grid or array arrangement are collated into a corresponding array or frame. Such a frame may be presented as an image to help visualise the output signals from a touchscreen panel as a whole.
FIG. 1 illustrates noise sources in a touchscreen panel. In grayscale plots illustrating frames Fsingle, F0, regions labelled “DB” correspond to low signal levels, regions labelled “LB” correspond to intermediate signal levels and regions labelled “R” correspond to high signal level.
Referring to FIG. 1, noise signals in a touchscreen panel may come from many sources including, for example, stochastic, or random noise components 1, deterministic, or common-mode noise components 2 and offsets 3. Stochastic noise components 1 represents random noise, for example, thermal noise from the touchscreen circuitry. Deterministic noise components 2 represents systematic sources, for example, mains electricity pick-up at 50-60 Hz, noise generated by a display, common-mode noise including power supply spikes induced by a charger for a mobile device. Deterministic noise components 2 encompasses regular but unintended signals in the touchscreen panel. Deterministic noise components 2 may be induced by other components of a device including a touchscreen panel, or may result from the ambient environment. The sum of the stochastic noise components 1 and deterministic noise components 2 produces the noise signal 4. In practice, it is often difficult or impossible to strictly divide the noise 4 into stochastic 1 and deterministic 2 components, and different frequency bandwidths may include greater or lesser contributions from stochastic 1 and deterministic 2 components. Offsets 3 may be non-uniform across the grid or array of the touchscreen panel. Offsets 3 may arise because the readout from an individual electrode, an intersection of electrodes, a sensor etc, may in general have a slightly different DC offset. Offsets 3 need not be DC, and in some touchscreen panels the offsets 3 may slowly vary or drift over time. Offsets 3 may result from the hardware of a device incorporating the touchscreen panel, or may result from factors in the ambient environment such as, for example, ambient temperature. The noise 4 combines with the offsets 3 to produce a background noise frame F0. When the touchscreen panel is touched, the resulting signal frame Fsingle is a superposition of the desired touch signals and the background noise frame F0.
FIGS. 2A, 2B and 2C illustrate touchscreen signal frames Fsingle, Fmulti, F0. In greyscale plots illustrating touchscreen panel frames Fsingle, Fmulti, F0, regions labelled “DB” correspond to low signal levels, regions labelled “LB” correspond to intermediate signal levels and regions labelled “R” correspond to high signal level.
Referring also to FIGS. 2A, 2B and 2C, the touchscreen signal frames may be single touch signal frames Fsingle, multi-touch signal frames Fmulti or no-touch signal frames, such as background noise frame F0. The term “single-touch” refers to a user interaction in which a single digit of a user, or a single suitable stylus touches or presses the touchscreen panel. The term “multi-touch” refers to a user interaction in which two or more digits of a user, or two of more suitable styluses, or a combination of a user's digits and a stylus, touch or press the touchscreen panel concurrently.
In an ideal touchscreen panel, the signal frames F0, Fsingle, Fmulti would register zero everywhere except at the locations of user interactions. However, in general, almost all of the locations of a two dimensional grid or array arrangement of a touchscreen will have non-zero values, so that signals resulting from touch events must be large enough to overcome the noise 4 and offsets 3.
Consequently, the signal-to-noise ratio (SNR) of the touchscreen panel may be compromised, resulting in failures to register touch events and/or false registration of touch events which have not actually occurred. In order to achieve a high SNR, the excitation power may be increased for an active touchscreen panel or the amplification gain may be increased for a passive touchscreen panel. However, increasing the excitation power or amplification gain may increase the power consumption of a touchscreen panel and shorten battery life of a device incorporating the touchscreen panel. In touchscreen panels which provide analog outputs, for example in pressure sensing touchscreen panels, the SNR may limit the resolution of analog output signals, for example, force-touch signals.
The offset 3 of a single electrode, electrode intersection, sensor etc in a touchscreen panel may be cancelled by subtracting a fixed value or by applying a detection threshold which must be exceeded for a touch event to be registered. However, non-uniformity of the offsets 3 across the touchscreen panel may make application of a single threshold or offset correction difficult, and may require a high threshold to avoid touch registration errors.
FIG. 3 illustrates touch signals obtained from a number of electrodes of a touchscreen panel.
Referring to FIG. 3, 1st to Nth electrode of a touchscreen panel have respective offset values Δ1, Δ2, . . . , ΔN. A detection threshold for registering a touch event should be set at a level which at least exceeds the largest offset value Δ1, Δ2, . . . , ΔN. For example, in a case when the output signals of a touchscreen panel are voltage signals corresponding to each electrode, a voltage threshold VTH should be set which at least exceeds the largest voltage offset value ΔV1, ΔV2, . . . , ΔVN. Setting a detection threshold VTH according to the largest offset value ΔV1, ΔV2, . . . , ΔVN may increase power consumption and may also reduce the dynamic range available for using analog output signals from, for example pressure sensing touchscreen panels.